Carpal tunnel !

The carpal tunnel is a narrow passage at the front of the wrist through which the median nerve and the finger flexor tendons reach the hand.

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What it is

A tunnel at the front of the wrist: the curved carpal bones form the floor and walls, the flexor retinaculum the roof (from scaphoid and trapezium to pisiform and hook of hamate).

What it does

Carries the median nerve and nine tendons into the hand: four of FDS, four of FDP and FPL. The ulnar nerve and artery and the median nerve's palmar branch pass above the roof, not through it.

Key facts

Where: Front of the wrist: carpal bones (floor and walls) under the flexor retinaculum (roof)
Attaches: Roof: scaphoid tubercle and trapezium (lateral) to pisiform and hook of hamate (medial)
Function: Contents: median nerve + 9 tendons (4 FDS, 4 FDP, FPL); not inside: ulnar nerve and artery, palmar cutaneous branch of the median nerve, FCR (own groove in the retinaculum)

Relations

Contents: Median nerve; FDS tendons in two rows (middle and ring in front, index and little behind); FDP tendons side by side behind them; FPL tendon on the radial side; the ulnar bursa around the finger flexors and the radial bursa around the FPL
Superficial: Flexor retinaculum, with the palmaris longus tendon and the palmar cutaneous branch of the median nerve on it; the ulnar nerve and ulnar artery cross on the ulnar side in Guyon canal
Deep: Floor of carpal bones covered by the palmar wrist ligaments, concave forwards; the capitate lies at the centre of the floor
Lateral: Scaphoid tubercle and trapezium; a septum from the deep surface of the retinaculum walls off the FCR tendon in the trapezium groove
Medial: Pisiform and hook of the hamate, the ulnar wall; the deep branch of the ulnar nerve curls round the hook just outside the tunnel
Below: The recurrent motor branch of the median nerve to the thenar muscles and the superficial palmar arch lie just beyond the distal edge, in the palm

How to find it

The tunnel lies under the base of the palm, from the distal wrist crease about 3 cm into the palm. Its four corners are felt as bony points: pisiform and hook of the hamate on the ulnar side, scaphoid tubercle and trapezium on the radial side. The median nerve runs roughly under the palmaris longus tendon, slightly to its radial side.

Examination

Phalen test: holding the wrists fully flexed raises the pressure in the tunnel; tingling in the thumb, index, middle and radial half of the ring finger within about a minute suggests carpal tunnel syndrome
Tinel sign over the tunnel and the Durkan compression test: tingling in the median nerve fingers on tapping or pressing over the tunnel supports compression there
Sensation over the thenar eminence is normal in carpal tunnel syndrome, because the palmar cutaneous branch runs outside the tunnel; loss there points to a lesion higher up the forearm

On imaging

Nerve conduction studies confirm and grade the compression. Ultrasound shows the median nerve in cross-section; an enlarged area at the tunnel inlet (pisiform level) supports compression. The carpal tunnel X-ray view (wrist extended, beam along the tunnel) shows the hook of the hamate and the floor. MRI shows the tendons, sheaths and any mass.

Common problems

Carpal tunnel syndrome: swelling in the tunnel squeezes the median nerve - night tingling in the thumb, index and middle fingers, later thumb weakness.
The palm stays normal to touch, because the palmar branch of the median nerve runs above the tunnel.

How it is diagnosed

Symptoms with tapping (Tinel) and wrist-bending (Phalen) tests; nerve conduction studies confirm it.

Treatment

Night wrist splint and sometimes a steroid injection.
Carpal tunnel release: cutting the flexor retinaculum to give the nerve room.

Clinical notes

Carpal tunnel syndrome is the most common nerve entrapment in the body; anything that fills the tunnel (tenosynovitis, pregnancy fluid, hypothyroidism, amyloid, a ganglion, fracture callus) or narrows it can cause it
Release divides the roof on its ulnar side, away from the recurrent motor branch; the superficial palmar arch just beyond the distal edge marks the limit
Acute carpal tunnel syndrome can follow a displaced distal radius fracture or a lunate dislocation, in which the lunate is pushed forward into the tunnel
Infection in the ulnar or radial bursa spreads through the tunnel to the forearm, because both sheaths run a few centimetres above the retinaculum

Anatomical variants

A persistent median artery runs with the median nerve in a minority of wrists, sometimes between the two parts of a bifid median nerve
The lumbrical origins can be drawn into the tunnel when the fingers are flexed, and low-lying FDS muscle bellies or an accessory muscle can crowd it

Carpal tunnel syndrome

Test yourself

Think of the answer, then tap the question to see it.

What forms the roof, the floor and the walls of the carpal tunnel?

The flexor retinaculum is the roof; the carpal bones and their palmar ligaments form the floor and walls, with the pisiform and hook of the hamate medially and the scaphoid tubercle and trapezium laterally.

Which ten structures pass through the carpal tunnel?

The median nerve and nine tendons: four of FDS, four of FDP and the FPL.

Why is the FCR tendon not counted among the contents of the carpal tunnel?

It runs in its own channel, split off by a septum of the flexor retinaculum against the trapezium groove.

Why is sensation over the thenar eminence spared in carpal tunnel syndrome?

The palmar cutaneous branch of the median nerve leaves above the wrist and passes superficial to the flexor retinaculum.

Which nerve and artery pass superficial to the carpal tunnel, and in which canal?

The ulnar nerve and ulnar artery, in Guyon canal between the pisiform and the hook of the hamate.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): canalis carpi TA2 2551
Official Spanish (DeCS): Conducto carpiano
English (TA2): Carpal tunnel
Also called: carpal canal

Sources: Terminologia Anatomica (TA2); Anatomía humana (4.ª ed.), tomo 1, cap. 54: Huesos del miembro superior; Anatomía humana (4.ª ed.), tomo 1, cap. 58: Mano; Anatomy, Shoulder and Upper Limb, Wrist Flexor Retinaculum.

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Representative model, for educational purposes only.